1. Technical Field
The present disclosure relates generally to phase change memories.
2. Description of the Related Art
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states, for electronic memory application. The state of the phase change materials is also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until changed by another programming event, as that value represents a phase or physical state of the material (e.g., crystalline or amorphous). The state is unaffected by removing electrical power.
A phase change memory device includes an array of memory cells, each memory cell comprising a memory element and a selection element. Both the memory element and the selection element may be made of a chalcogenide material. The memory element and the selection element may be sandwiched between a lower electrode and an upper electrode. Select devices may also be referred to as an access device, an isolation device, or a switch.
Programming of the phase change material to alter its state or phase may be accomplished by applying voltage potentials across the electrodes, thereby generating a voltage potential across the select device and the memory element. When the voltage potential is greater than the threshold voltages of the select device and the memory element, an electrical current may flow through the phase change material in response to the applied voltage potentials, and may result in heating of the phase change material.
This heating may alter the memory state or phase of the phase change material, thus altering the electrical characteristic of the memory material, e.g., the resistance. Thus, the memory material may also be referred to as a programmable resistance material.
The amorphous or semi-amorphous state may be associated with a “reset” state or a logic “0” value, while a crystalline or semi-crystalline state may be associated with a “set” state, or a logic “1” value. The resistance of memory material in the amorphous or semi-amorphous state is generally greater than the resistance of memory material in the crystalline or semi-crystalline state. It is to be appreciated that the association of reset and set with amorphous and crystalline states, respectively, is a convention and that at least an opposite convention may be adopted.
Using an electrical current, the memory material may be heated to a relatively higher temperature to amorphize memory material and “reset” memory material (e.g., program memory material to “0”). Heating the volume of memory material to a relatively lower crystallization temperature may crystallize memory material and “set” memory material (e.g., program memory material to “1”). Various resistances of memory material may be achieved to store information by varying the amount of current flow and duration through the volume of memory material.
Referring to FIG. 1, a conventional phase change memory cell 100 includes a phase change layer 110 and an ovonic threshold switch (OTS) 106 positioned between a top and a bottom electrode, 104 and 112 respectively. A middle electrode 108 is located between the phase change layer 110 and the OTS 106. The top electrode 104 is aligned with a bitline 102 and separates the bitline 102 from the OTS 106. The phase change layer 110 is in direct contact with the bottom electrode 112. In this arrangement, the heat from the phase change layer 110 escapes through the bottom electrode 112, which is in direct contact with a wordline 114. The wordline 114 acts as a thermal sink drawing away heat that could otherwise be used for altering the phase of the phase change layer 110. Therefore, the programming current must be increased to achieve a temperature sufficient to program the phase change layer 110.